A long term goal of our laboratory is to characterize how alterations in mechanical load affect cellular mechanisms of muscle contraction. Resistance exercise training is a commonly prescribed modality for altering skeletal muscle function. However, training induced alterations in human muscular strength and performance may be due to changes in neural mechanisms of motor unit recruitment as well as to changes occurring within the muscle per se. Because of the complexity of the human neuromuscular system, it has been difficult to define the specific cellular mechanisms of contraction that are affected by resistance exercise training. We propose using an in vitro, single cell preparation to examine alterations in human skeletal muscle fiber function following 12 weeks of resistance exercise training. Single muscle fibers will be isolated from pre- and post-training vastus lateralis muscle biopsies, chemically skinned to render the sarcolemma permeable to Ca2+, and mounted between a force transducer and servo-controlled position motor. Peak force, maximal shortening velocity, and a force-velocity-power relationship will be determined for each fiber under conditions of maximal Ca2+- activation. Following the physiological experiments, gel electrophoresis will be used to determine the myosin heavy chain and light chain composition of each fiber. The results of this study will provide important insight into the relationships between protein isoform expression and cell function and how these relationships are altered by resistance exercise.